Retention Of 14C Research Articles

Behaviour of carbon-14 containing low molecular weight organic compounds in contaminated groundwater under aerobic conditions

Short chain carbon-14 (14C) containing organic compounds can be formed by abiotic oxidation of carbides and impurities within nuclear fuel cladding. During fuel reprocessing and subsequent waste storage there is potential for these organic compounds to enter shallow subsurface environments due to accidental discharges. Currently there is little data on the persistence of these compounds in such environments. Four 14C-labelled compounds (acetate; formate; formaldehyde and methanol) were added to aerobic microcosm experiments that contained glacial outwash sediments and groundwater simulant representative of the Sellafield nuclear reprocessing site, UK. Two concentrations of each electron donor were used, low concentration (10−5 M) to replicate predicted concentrations from an accidental release and high concentration (10−2 M) to study the impact of the individual electron donor on the indigenous microbial community in the sediment. In the low concentration system only ∼5% of initial 14C remained in solution at the end of experiments in contact with atmosphere (250–350 h). The production of 14CO2(g) (measured after 48 h) suggests microbially mediated breakdown is the primary removal mechanism for these organic compounds, although methanol loss may have been partially by volatilisation. Highest retention of 14C by the solid fractions was found in the acetate experiment, with 12% being associated with the inorganic fraction, suggesting modest precipitation as solid carbonate. In the high concentration systems only ∼5% of initial 14C remains in solution at the end of the experiments for acetate, formate and methanol. In the formaldehyde experiment only limited loss from solution was observed (76% remained in solution). The microbial populations of unaltered sediment and those in the low concentration experiments were broadly similar, with highly diverse bacterial phyla present. Under high concentrations of the organic compounds the abundance of common operational taxonomic units was reduced by 66% and the community structure was dominated by Proteobacteria (particularly Betaproteobacteria) signifying a shift in community structure in response to the electron donor available. The results of this study suggest that many bacterial phyla that are ubiquitous in near surface soils are able to utilise a range of 14C-containing low molecular weight organic substances very rapidly, and thus such substances are unlikely to persist in aerobic shallow subsurface environments.

COMPARATIVE BIOKINETICS OF RADIOCARBON INGESTED AS COMPOUNDS OR FOODS IN RATS

The biokinetics of radiocarbon (¹⁴C) in rats continuously ingested in the forms of amino acids (glycine and leucine) or as foods (wheat and rapeseed) were examined and compared with each other. The differences in the biokinetics between ¹⁴C-amino acids and ¹⁴C-foods were observed in the excretion of ¹⁴C in feces and in the incorporation and retention of ¹⁴C in the rat tissues. At the 10 week of continuous ingestion of ¹⁴C-amino acids, the integrated activities of ¹⁴C in all the tissues were within 5% of the ingested activity. The integrated activities in each tissue coming from ¹⁴C-food ingestion were higher by a factor of more than 2 compared to those coming from ¹⁴C-amino acid ingestion. Radiation dose rates estimated at the 10 week showed that the dose rates to the individual tissues coming from ¹⁴C-food ingestion were 2-5 times higher than those coming from ¹⁴C-amino acid ingestion. The difference in the dose rate between ¹⁴C-amino acid and ¹⁴C-food was more significant in adipose tissue by a factor of 7-30.

Biokinetics and radiation dosimetry for patients undergoing a glycerol tri[1- 14C]oleate fat malabsorption breath test

The glycerol tri[1- 14C]olein test for fat malabsorption was carried out in two male volunteers and measurements of the loss of 14C in expired air, urine and faeces and the retention of 14C in biopsy samples of abdominal fat were made using accelerator mass spectrometry. Exhalation accounted for 73% and 55% of the administered activity and could be described by three-component exponential functions with halftimes of about 1 h, 2 days and 150 days, respectively. Urinary excretion accounted for 24% of the administered activity, almost all during the first 24 h after administration; about 2% was excreted in the faeces in 48 h. The halftime of retention of 14C in fat ranged from 137 to 620 days. Absorbed dose calculations indicate that for a normal adult the largest dose, 1.5–7.0 mGy/MBq is received by the adipose tissue, and that the effective dose is 0.3–0.5 mSv/MBq. It is concluded that no restrictions need to be placed on radiation safety grounds on the administration of 0.05–0.1 MBq 14C-triolein for the triolein breath test.

Glucose and pyruvate metabolism during mouse gonadal sex differentiation.

In the mouse, gonadal sex differentiation starts around E12 and meiosis begins in the ovary shortly after E13. In the search for metabolic changes that might be correlated to gonadal sex differentiation and/or possibly the onset of meiosis, we investigated the metabolism of glucose and pyruvate in the developing mouse ovary before (E11.5-E12.5), during (E14.5-16.5), and after meiosis (E18.5), and in fetal testes without meiosis. Gonads were cultured with 14C-labeled glucose (0.02 and 5.58 mM) and 14C-pyruvate (0.17 mM). The oxidation expressed as 14CO2 production and the organification expressed as retention of 14C in the tissues were measured and correlated to the protein content of the gonads. Using 0.02 mM glucose, a decline in oxidation and organification was found in ovaries as well as in testes, which is probably related to starvation. Using 5.58 mM glucose, a continuous decline in oxidation was seen only in the testis. Organification of 0.17 mM pyruvate increased at E12.5 and E14.5 in the ovary but not in the testis. This was in despite of an exponential increase of protein content in the testes compared to only a moderate increase in the ovary. The CO2 production from 5.58 mM glucose was equal to that from 0.17 mM pyruvate in gonads of both sexes. In conclusion, an increased metabolism of 5.58 mM glucose and 0.17 mM pyruvate in the ovaries as compared to the testes is related to sex differences during gonadal formation and onset of meiosis in the ovaries. J. Exp. Zool. 288:130-138, 2001.

Glucose and pyruvate metabolism during mouse gonadal sex differentiation

In the mouse, gonadal sex differentiation starts around E12 and meiosis begins in the ovary shortly after E13. In the search for metabolic changes that might be correlated to gonadal sex differentiation and/or possibly the onset of meiosis, we investigated the metabolism of glucose and pyruvate in the developing mouse ovary before (E11.5-E12.5), during (E14.5-16.5), and after meiosis (E18.5), and in fetal testes without meiosis. Gonads were cultured with 14C-labeled glucose (0.02 and 5.58 mM) and 14C-pyruvate (0.17 mM). The oxidation expressed as 14CO2 production and the organification expressed as retention of 14C in the tissues were measured and correlated to the protein content of the gonads. Using 0.02 mM glucose, a decline in oxidation and organification was found in ovaries as well as in testes, which is probably related to starvation. Using 5.58 mM glucose, a continuous decline in oxidation was seen only in the testis. Organification of 0.17 mM pyruvate increased at E12.5 and E14.5 in the ovary but not in the testis. This was in despite of an exponential increase of protein content in the testes compared to only a moderate increase in the ovary. The CO2 production from 5.58 mM glucose was equal to that from 0.17 mM pyruvate in gonads of both sexes. In conclusion, an increased metabolism of 5.58 mM glucose and 0.17 mM pyruvate in the ovaries as compared to the testes is related to sex differences during gonadal formation and onset of meiosis in the ovaries. J. Exp. Zool. 288:130-138, 2001.

Diffusion and degradation of atrazine in a water/sediment system

AbstractThis study determined the effects of temperature and residence time on diffusion and degradation of [14C]atrazine applied to the surface of sterilized and nonsterilized river water over a sediment. Water and sediment were placed in glass columns and incubated at 5 or 24°C. Samples were collected at 0, 7, 14, 28, 42, 56, and 112 d before and after disturbance. The warmer temperature (24°C) initially increased the rate of 14C distribution in the aqueous phase. Disturbance caused approximately 15% of the total 14C to be removed from the aqueous phase by sterile or unsterile sediment incubated at 5°C. At 24°C, the level of 14C in disturbed sediments incubated for more than 28 d gradually increased. This increased retention of 14C by sediment coincided with greater detection of the polar metabolite hydroxyatrazine (HA). More HA was recovered in nonsterilized than in sterilized sediments, indicating the contribution of biological processes to the production of this metabolite. In undisturbed sediments at 24°C, approximately 32% and 10% of the applied 14C permeated the top and bottom 1‐cm segments, respectively. In columns incubated at 5°C, less than 5% of the 14C reached the surface of the submerged sediment. The potential impact of atrazine on biota associated with sediment would diminish with time because of dilution in the aqueous phase, transformation to HA, and bound residue formation.

DIFFUSION AND DEGRADATION OF ATRAZINE IN A WATER/SEDIMENT SYSTEM

This study determined the effects of temperature and residence time on diffusion and degradation of [14C]atrazine applied to the surface of sterilized and nonsterilized river water over a sediment. Water and sediment were placed in glass columns and incubated at 5 or 24°C. Samples were collected at 0, 7, 14, 28, 42, 56, and 112 d before and after disturbance. The warmer temperature (24°C) initially increased the rate of 14C distribution in the aqueous phase. Disturbance caused approximately 15% of the total 14C to be removed from the aqueous phase by sterile or unsterile sediment incubated at 5°C. At 24°C, the level of 14C in disturbed sediments incubated for more than 28 d gradually increased. This increased retention of 14C by sediment coincided with greater detection of the polar metabolite hydroxyatrazine (HA). More HA was recovered in nonsterilized than in sterilized sediments, indicating the contribution of biological processes to the production of this metabolite. In undisturbed sediments at 24°C, approximately 32% and 10% of the applied 14C permeated the top and bottom 1-cm segments, respectively. In columns incubated at 5°C, less than 5% of the 14C reached the surface of the submerged sediment. The potential impact of atrazine on biota associated with sediment would diminish with time because of dilution in the aqueous phase, transformation to HA, and bound residue formation.

Distribution of [14C] Dichlorophenoxyacetic Acid in Cultured Zygotic Embryos of Zea mays L.

The uptake of 2,4-dichlorophenoxyacetic acid (2,4-D), necessary for the in vitro induction of callus formation and somatic embryogenesis in cultured immature maize embryos, was quantified after culture on nutrient medium with [14C]2,4-D. The identity of the 14C label in the embryos was determined by high performance liquid chromatography (HPLC), and its distribution within embryos was visualized on sections of plastic embedded material. Quantification of the 14C label after a pulse label of 16 h showed a hundredfold accumulation of 2,4-D in the embryos with respect to the initial medium concentration. During tissue processing for in situ detection of 14C, however, up to 70% of the label disappeared because of the embedding process. The best structural preservation was obtained after ethanol-mediated infiltration of Technovit 7100. Water-mediated infiltration of Technovit 7100 gave the highest retention of 14C. HPLC analysis showed that more than 95% of the residual 14C label found in embryos was still 2,4-D. Autoradiography showed that the embryogenic inbred line A188 contained 14C label in distinct regions of the scutellum, coleoptile, and suspensor. The nonembryogenic inbred line A632 contained more label after 16 h of culture in a different distribution compared with A188. Subculture of the embryos for 24 and 72 h and histologic analysis showed that cell proliferation and callus formation were restricted to specific regions of the embryo in both inbred lines. The pattern of 2,4-D distribution did not codistribute with regions of proliferation, indicating that 2,4-D is not the only trigger for proliferation.

Uptake of arachidonic acid from blood 2-lyso-phosphatidylcholine by extrahepatic tissues in vivo: An experimental study in the rat

In previous work, we demonstrated that part of arachidonic acid (AA, n-6) pools in the gastrointestinal tract and blood forming tissues could be formed by local interconversion of linoleic acid taken up as free fatty acid from blood. In the rat the liver releases significant amounts of 2-arachidonyl-lysophosphatidylcholine (2-AA-LPC). In this study, we took advantage of the high hydrolytic activity of recombinant guinea pig pancreatic lipase-related protein 2 against the 1-ester bond of glycerophospholipids to conveniently prepare 2-[ 14C]AA-LPC by hydrolysis of radioactive phosphatidylcholine (L-α-1-Palmitoyl-2-arachidonyl [arachidonyl-1- 14C]) and examined the rate of uptake of 2-[ 14C]AA-LPC by the tissues in rats. Albumin-bound 2-[ 14C]AA-LPC was injected intravenously into male rats. The clearance rate of the 2-[ 14C]AA-LPC was fast, the initial half-life being 1.07 min. At 10 min after injection, the retention of 14C was 2.0–3.3%/g tissue of injected dose in the liver, 0.4 to 0.6%/g in stomach, 1.1 to 1.3%/g in small intestine, 0.4 to 0.6%/g in colon, 0.3 to 0.4%/g in bone marrow, and 0.7 to 1.0%/g in spleen. In most tissues, the proportion of radioactivity in tissue lipids was highest in PC (51 to 72%). In the heart, lung, and spleen the radioactivity in LPC was most predominant. The rate of uptake of AA as LPC by the liver was 61.0 ± 7.3 nmol/min, and by the gastrointestinal tract including stomach, small intestine and colon was 25.0 nmol/min. This study provides a quantitative estimation of the role of 2-AA-LPC in the supply of AA to the AA pools in extrahepatic tissues. It indicates that uptake of liver derived 2-AA-LPC may be one of the most important AA sources for extrahepatic tissues in the rat, particularly the gastrointestinal tract.

Tissue uptake and interconversion of plasma unesterified 14C linoleic acid in the guinea pig

Part of the arachidonic acid (20 : 4, n−6) pools in the gastrointestinal tract and blood forming tissues may be formed by local interconversion of linoleic acid (18 : 2, n−6) taken up as a free fatty acid from blood. This study examined the rate of uptake and interconversion of unesterified 14C-18 : 2 by different tissues in young guinea pigs. The clearance rate of 14C-18 : 2 was fast, and the initial half-life was 6.3 s. The retention of 14C in tissue lipids was 1.6–1.8% g −1 in the liver, 0.4% g −1 in stomach, 0.7% g −1 in small intestine, 0.2% g −1 in colon, 0.4% g −1 in bone marrow and 0.7% g −1 in spleen. Autoradiographic localization of 3H-18 : 2 under light microscope demonstrated that most of the 3H radioactivity of the gastrointestinal tract was in the mucosa, in both villus and crypt cells. In bone marrow smears, a high density of silver grains was found in megakaryocytes. The percent of 14C in δ6 desaturase products was higher in gastrointestinal tract, heart, lung, bone marrow and spleen than in liver. The ratio of 14C-20 : 3 / 14C-20 : 4 formation in most tissues was high, and a notable finding being a lower rate of 20 : 4 formation from plasma free 18 : 2 in the liver, (170 pmol min −1) than in the gastrointestinal tract (428 pmol min −1) and bone marrow (1203 pmol min −1). The local interconversion of 18 : 2 into δ6 desaturase products is thus an important source of 20 : 4 in these organs in guinea pigs.

Measurement of protein turnover in the small intestine of lambs. 1. Development of an experimental model

The objective of this study was to develop an experimental model to measure both fractional and absolute rates of protein synthesis in the small intestine of lambs. Six male castrate lambs (∼6 months old, mean liveweight 26 kg) were offered, via continuous feeders, 900 g/day lucerne chaff. They were prepared with catheters in the cranial mesenteric vein (CMV), femoral artery (FA), jugular vein and abomasum, and a blood flow probe around the CMV. Cr-EDTA (0·139 mg Cr/ml, ∼0·2 ml/min) was infused abomasally for 24 h and L-[2,6-3H]phenylalanine (Phe) (441±33·8 μCi into the abomasum) and L-[U-14C]phenylalanine (43·9±4·08 μCi into the jugular) were also infused during the last 8 h. Blood from the CMV and FA was sampled during isotope infusions. At the end of infusions, lambs were killed and tissue and digesta samples removed from four sites along the small intestine (SI). Transfers of labelled and unlabelled Phe were measured between SI tissue, its lumen and blood, enabling both fractional and absolute rates of protein synthesis and gain to be estimated. The total SI protein pool was 84 (±1·7) g and fractional gain rate was 7·5 (±5·5)% per day. Mean protein synthesis and fractional synthesis rates (FSR) were calculated from the mean retention of 14C and 3H in SI tissue. FSR tended to increase caudally along the SI (although P > 0·05). The choice of free Phe pool for estimating precursor specific radioactivity (SRA) for protein synthesis had a significant effect on FSR. Assuming that tissue free Phe SRA represented precursor SRA gave a mean FSR of 129 (±24)% per day. Corresponding estimates for free Phe SRA in the FA and CMV were 20 (±1·8) and 30 (±3·1)% per day respectively. The correct value for protein synthesis was therefore in doubt, although indirect evidence suggested that blood SRA (either FA or CMV) may be closest to true precursor SRA. This evidence included (i) comparison with flooding dose estimates of FSR, (ii) comparison of 3H[ratio ]14C Phe SRA in free Phe pools with this ratio in SI protein, and (iii) the proportion of SI energy use associated with protein synthesis. Advantages of the present experimental model compared to other methods included (i) measurements of both protein synthesis and gain, and hence, all components of turnover, (ii) measurement of absolute as well as fractional rates of synthesis and gain, (iii) inclusion of proteins which are synthesised and exported, and (iv) concurrent measurement of protein synthesis and energy utilization by the small intestine.

Retention of Inorganic Carbon‐14 by Isotopic Exchange in Soils

AbstractCarbon‐14 is present naturally and is a waste product of the nuclear industry. It is readily incorporated into biological materials and potential impacts have to be considered carefully. From underground waste repositories, much of any 14C released will be inorganic and will pass rapidly through most terrestrial environments because of gaseous transfer. However, there will be some 14C retained in soils, in both organic and inorganic forms. This study addresses retention of inorganic 14C through isotopic exchange with soil carbonates. An outdoor study with incubations lasting up to 24 mo was conducted. A carbonated sandy soil was used unamended, amended with additional carbonate, or amended with ground alfalfa (Medicago sativa L.). One set of soil containers was left open and another set gamma‐sterilized and sealed. Volatilization of some of the 14C happened within minutes, with 98% lost in the alfalfa‐amended soils and 56% lost in the carbonate‐amended soil. However, the fractions remaining in all the soils had residence half‐times of over 1000 d. Much of this retained 14C was still inorganic and had undergone isotopic exchange with the solid 12C minerals in the soil. A progressive extraction method was used to monitor the degree of isotopic exchange, and in certain treatments the exchange process was still measurably progressing after 12 mo of incubation. Clearly, assessments of the impacts of 14C releases to the terrestrial environment must account for these long‐term reactions and retention of 14C in soils.

Uptake and interconversion of plasma unesterified 14C linoleic acid by gastrointestinal tract and blood forming tissues: An experimental study in the rat

The origin of the arachidonic acid (20:4, n-6) in the gastrointestinal tract and blood forming tissues is unknown. This study examines the rate of uptake and interconversion of unesterified 14C linoleic acid (18:2, n-6) by the liver, gastrointestinal tract, bone marrow, and spleen. Albumin-bound unesterified 14C-18:2 was injected intravenously into young male rats. The clearance rate of the 14C-18:2 and the mass concentration of unesterified 18:2 in plasma were measured. After 5 and 10 min, the radioactivity of the tissue lipids and the degree of interconversion of 14C-18:2 were determined. The rate of retention and interconversion of plasma unesterified 18:2 in different tissues could then be calculated. The retention of 14C in liver lipids (3.6 to 4.5%/g of tissue) was several-fold higher than in the stomach (0.4%/g), small intestine (0.3%/g), colon (0.2%/g), and spleen (0.2%/g). Higher proportions of the 14C were in phospholipids (PL) in the extrahepatic tissues (50 to 80%) than in the liver (42 to 44%). The main part of the PL radioactivity was in phosphatidylcholine. The percent of labeling in cardiolipin was 4 to 10 fold higher in the gastrointestinal tract and 8 to 20 fold higher in the spleen and bone marrow than in the liver. The percent of interconversion of retained 14C-18:2 to δ6 desaturase products was higher in colon (16%), spleen (18%), and bone marrow (15 to 16%) than in liver (10 to 11%). The rate of 20:4 formation from plasma-free 18:2 was 14,200 pmol/min in liver, 106 pmol/min in the stomach, 751 pmol/min in the small intestine, and 159 pmol/min in the colon. This study indicates that a large part of the 20:4 pools in the gastrointestinal tract and blood-forming tissues can be formed by local interconversion reactions.

Metabolic disposition in rats of regular and enzymatically depolymerized sodium carboxymethylcellulose

Partially enzyme-hydrolysed sodium carboxymethylcellulose (CMC-ENZ), which holds promise as a new, functional food ingredient, is obtained from sodium carboxymethylcellulose (CMC) by enzymatic hydrolysis with a cellulase preparation from Trichoderma longibrachiatum. In the safety evaluation of CMC-ENZ, a comparative disposition study on 14C-labelled CMC and CMC-ENZ was conducted in conventionally kept rats. The 14C label was in the two C atoms of the carboxymethyl group. Two groups of four male and four female rats each were fed diets with 5% unlabelled CMC or CMC-ENZ for a 2-wk adaptation period. A single oral dose of 14C-CMC or 14C-CMC-ENZ solution was then given by gavage (500 mg/kg body weight). Respiratory CO 2, urine and faeces were collected at regular intervals, and after 120 hr organs, tissues and the carcass were sampled as well. For both experimental groups, total mean recovery of 14C was 98%, about 95% of the label being excreted with the faeces, 2% or less in the urine, 1% or less with CO 2 and a small fraction being retained in the body (CMC, 0.58%; CMC-ENZ, 0.75%). Tissue retention of 14C was highest in the liver of rats of both experimental groups. Only about 49 and 65% of the faecal 14C was extracted with water in the 14C-CMC and 14C-CMC-ENZ dosed rats, respectively. Gel permeation chromatography (GPC) of the dosing solutions and the faecal extracts revealed that CMC is depolymerized during intestinal passage whereas CMC-ENZ is excreted nearly unchanged. Consequently, the molecular weight distribution of the 14C-CMC and 14C-CMC-ENZ faecal excretion products was similar. It is concluded that there is no toxicologically relevant difference between the disposition of CMC and CMC-ENZ.

Photosynthate Reserves during Grain Filling in Winter Wheat

AbstractHow much does stored or reserve photosynthate contribute to grain growth in winter wheat (Triticum aestivum L.)? Three models were developed to assess this contribution. In Model I, all current photosynthate was used for vegetative or reproductive growth, and there was no reserve. In Model II, photosynthate accumulated in a reserve was used preferentially to support maintenance respiration, while current photosynthate accumulated in the grain. In Model III, the maintenance component extracted from the reserve was continuously replenished from current photosynthate, thus requiring a greater contribution from the reserve for dry matter in the grain. Each model related dry matter accumulation at the whole‐plant level to photosynthesis, respiration, and the distribution and retention of 14C from 14CO2 assimilated photosynthetically. To test the models, these characteristics were measured in four cultivars of New York soft white winter wheat grown in the field in Connecticut in three years. Observations showed that canopy respiration and grain dry matter accumulation were approximately equal sinks for photosynthate and, together, were greater than canopy photosynthesis late in grain filling. Consequently, in two years, Model I (with no reserve) terminated grain filling early and predicted a lower harvest index than was observed. The observed time course of the distribution and retention of radioactivity from a pulse of 14C02 assimilated photosynthetically was most consistent with predictions of Model III (continuous replenishment of the reserve). In this model, preanthesis reserves contributed from 0.2 to 0.5 of grain dry weight of winter wheat in Connecticut, depending on harvest index. This contribution is expected to exceed 0.5 in more stressful climates.

Bioavailability of pyridoxine in rats fed various levels of casein

[ 14C] pyridoxine (PN) was orally administered to male rats and lactating dams fed an 8%, 20%, or 50% casein diet. There was no effect of the level of dietary protein on plasma pyridoxal 5′-phosphate (PLP) concentration. Analysis of fecal radioactivity indicated no dietary effect on the intestinal absorption of [ 14C]PN. There was greater retention of 14C in the liver when male and lactating rats were fed increased amounts of protein. Increased dietary protein resulted in reduced levels of 14C in the brain of the male rats. There was no dietary effect on the urinary excretion of total 14C or [ 14C]4-pyridoxic acid. Increasing the level of dietary protein resulted in increased retention of 14C in the liver of the lactating dams, however, the concentration of 14C and the 14C:protein ratio in milk was not affected. Although not significant, the level of 14C in the liver of the suckling pups was two or three times greater when dams were fed 8% protein than when dams were fed 20% or 50% protein. These results demonstrate that altered tissue distribution of vitamin B-6 occurs with varied protein intake without influencing the metabolic utilization and urinary excretion of vitamin B-6 or plasma PLP concentration, the primary assessment parameter for vitamin B-6 status. Furthermore, increased intake of protein by the lactating rat does not influence the concentration vitamin B-6 or the vitamin B-6:protein ratio in milk.

Cement-based engineered barriers for carbon-14 isolation

This paper presents a review of near-field investigations relevant to the use of cement-based engineered barriers for the long-term isolation of Ontario Hydro's 14C reactor operating waste. Specifically, the research program was designed to evaluate the effectiveness of cemented waste form and cementitious grout backfill for radiocarbon waste isolation in a repository setting. Carbonation of grout results in a calcitic end-product which has the potential to immobilize radio-carbon. 14C waste form evaluation studies indicate almost a total stoichiometric transfer of labile carbonate to the solid carbonate phase upon immobilization of carbonate-form resin in a cementitious matrix. The observed 14C leaching behaviour can be attributed to the dissolution characteristics of calcite in the waste form. The chemical buffering of the near-field groundwater environment by the cementitious matrix will ensure low solubility of calcite, thereby contributing to the retention of 14C in the waste form. Although the carbonation capacities of the backfill materials studied range from 16% to 19% of the grout mass, this high degree of carbonation was not attained for solid grout samples. Carbonation of grout not only contributes to the effectiveness of the engineered barrier as a geochemical sink for 14C but also causes a significant reduction in the permeability, resulting in diffusion-controlled mass transfer.

A comparison of seasonal patterns of photosynthate production and use in branches of red spruce saplings at two elevations

Seasonal patterns of carbon transport from 14C-labeled foliage were examined in red spruce (Picearubens Sarg.) branches at 1935 and 1720 m elevations in the Great Smoky Mountains National Park to characterize possible physiological mechanisms underlying the observed reductions in growth at high elevations. Current and 1-year-old foliage was labeled on 5- to 7-year-old branches of sapling-sized trees in June, August, and October. Retention of 14C was measured after 24 h, and allocation to other branch segments was characterized after 7 days. Total 14C-labeled carbon assimilation was greater in current than in 1-year-old foliage throughout the growing season. This suggests that photosynthetic capacity is high in developing foliage prior to needle maturity and remains high during the first growing season. In August, 14C retention in labeled foliage was less at the higher than at the lower elevation site, possibly reflecting increased respiratory losses that had been previously reported at the higher site. Retention of 14C in current-year shoots as a percent of 14C remaining after 7 days, a measure of sink strength, was high at both sites in June and remained high throughout the season in current-year shoots at the higher elevation site. The relatively high sink activity coupled with the large apportionment of biomass to current-year shoots (19.9 and 26.7% of total branch biomass at the lower and higher elevation sites, respectively) may make red spruce particularly susceptible to stresses such as winter injury that affect young foliage. In addition, the relatively greater sink activity of current-year shoots at the higher elevation site in August and October suggests that the contribution of current-year foliage to overall tree growth may be less at the higher than at the lower elevation site.

Photosynthetic Response of Wheat to Soil Water Deficits in the Tropics

AbstractThe changes in photosynthetic rate and translocation of photosynthates in winter wheat (Triticum Aestivum L.) grown in lysimeters were studied, in response to periodic soil water deficit during late tillering and flowering stages. Soil water deficits were imposed to previously nonstressed plants during late tillering and flowering states. Timing of irrigation was scheduled according to the ratio between irrigation water applied and cumulative pan evaporation (IW/CPE) of 0.75 (low deficit), and 0.5 (moderate deficit), as well as by suspending irrigations after crown root initiation stage (severe deficit). To determine the rate of photosynthesis, a short radioactive pulse of 14CO2 with 300 ppm concentration was given to second leaf from the top at tillering, and to the flag leaf at flowering stages for 20 second exposure time. The translocation of photosynthates was estimated by scanning 14C activity in different plant parts. In late tillering the midday Photosynthetic rate (PR) was significantly 3 mg CO2 dm−2 h−1 lower under low water deficit (WD1) than under zero water deficit (WD0). Under higher stress conditions, soil water acted as a limiting factor to keep the rate from rising above 13.2 during stress at late tillering (WD2), 14.5 flowering (WD4), and 10.0 mg CO2 dm−2 h−1 for stress at both the growth stages (WD5), respectively. The difference in daily accumulated photosynthesis (8 h), between stressed and nonstressed were 15, 40, 42, and 77 mg CO2 dm−2 h−1 respectively at WD1 WD2, WD4, and WD5. The retention of 14C in flag leaf decreased considerably after 24 hours of exposure time when the labelled assimilates were translocated in bulk to the ear head. Under stressed condition a general trend was observed for upward translocation of assimilates towards the ear, even from the stem and root. The percent 14C activity observed in ear after 24 hours was greatest in severely stressed plants. The photosynthetic rate is reasonable predicted by midday LDR and surface moisture.

Influence of Adsorption to Carbon Black Particles on the Retention and Metabolic Activation of Benzo[a]Pyrene in Rat Lungs Following Inhalation Exposure or Intratracheal Instillation

AbstractThere is a need to define the influence of adsorption of organic compounds onto particles on the biological fate of these compounds when inhaled. In this study, rats were exposed by nose-only inhalation (2 h) to 2 mg/m3 or 20 mg/m3 of pure [14C]BaP or [14C]BaP adsorbed onto carbon black particles at 0.2 mg/m3, 2.0 mg/m3, or 20 mg/m3 (total mass concentration ≃ 100 mg/m3). In separate studies, rats were intratracheally instilled with amounts of pure [14C]BaP and [14C]BaP on carbon black that were similar to those deposited in lungs by inhalation. In all cases, clearance of 14C from lungs had a rapid short-term component and a slower long-term component of clearance. As a percentage of the calculated initial lung deposition after inhalation or instillation, the long-term retention of 14C was similar for all amounts of pure [14C]BaP used and also similar for both methods of administration. The long-term lung retention of 14C as a percentage of the initial lung deposition was increased 16–60 times as ...